This invention relates to a process for producing microporous or open ultrafiltration poly(tetrafluoroethylene-co-perfluoro (alkyl vinyl ether)) (PFA) or poly (tetrafluoroethylene-co-hexafluoropropylene) (FEP) membranes under conditions to control membrane surface porosity to improve membrane permeability and to the membrane so-produced.
Microporous and open ultrafiltration membranes include thin sheets and hollow fibers generally formed from synthetic thermoplastic materials and having a substantially continuous matrix structure containing open pores or conduits of small size. The mean pore size range for pores of "microporous and open ultrafiltration membranes" is not precisely defined in the art, but it is generally understood to extend from about 0.02 microns to about 10 microns. Microporous and open ultrafiltration membranes having open pores thereby imparting permeability are useful in fine filtration.
PFA and FEP polymers are desirable filtration membrane materials because of their excellent chemical and thermal stabilities. However, their inherent inert nature also renders them unamenable to be cast into membranes by conventional solution immersion casting processes. Currently microporous membranes using similarly inert materials are disclosed in U.S. Pat. Nos. 3,953,566; 3,962,153; 4,096,227; 4,110,392; 4,187,390; 4,248,924; 4,482,516 and 4,598,011. The process disclosed in these patents comprises stretching sintered poly(tetrafluoroethylene) (PTFE) particles to create a pore structure characterized by nodes interconnected by fibrils. The pores are highly elongated in the stretch direction.
U.S. Pat. Nos. 4,623,670 and 4,702,836 disclose a process for forming microporous membranes from a fluoropolymer resin selected from the group consisting of ethylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer and poly(chlorotrifluoroethylene). In this process, an inorganic filler is required in melt molding the polymer with a chlorotrifluoroethylene oligomer. The filler and oligomer are dissolved out of the polymer to form voids. The use of fillers in microporous membranes used in filtration is highly undesirable since all of the fillers cannot be removed by solvation and the fillers remaining may migrate into the filtrate and contaminate it. Although the three fluoropolymers disclosed by these patents have good chemical and thermal resistance, they are inferior in stabilities when compared to PFA and FEP.
A method for making a porous fluorinated polymer structure is disclosed in U.S. Pat. No. 4,434,116. It involves forming a solvated or partially solvated polymer/solvent mixture. The polymer for which this is applicable comprises a copolymer of tetrafluoroethylene and perfluoro vinyl ether with a sulfonyl fluoride (--SO.sub.2 F), sulfonate (--SO.sub.3 Z) or carboxylate (--COOZ) functional group wherein Z is a cation. The presence of the polar functional group greatly enhances the dissolution of this polymer. A variety of organic solvents have been reported. The method described is based on thermal phase separation of the polymer/solvent mixture and specified that the solvent (porogen) is a solid at room temperature and must crystallize after phase separation. The solvent is then removed from the blend in the solid state. No pore morphology or permeability data of the porous structure were given.
The use of stretching technique as a primary means to form microporous membranes has been described in U.S. Pat. Nos. 3,953,566; 3,962,153; 4,096,227; 4,110,392 4,187,390; 4,248,924; 4,482,516 and 4,598,011. The process disclosed in these patents comprises stretching sintered poly(tetrafluoroethylene) (PTFE) particles to create a pore structure characterized by nodes interconnected by fibrils. No dissolution of polymer in solvent or thermal phase separation is involved in the membrane formation scheme.
The combination use of thermal phase separation from a melt blend (solution) and subsequent stretching (orientation) to form microporous membranes was described in U.S. Pat. No. 4,539,256. In this process, between 30 and 80% of a thermoplastic polymer is mixed with a solvent at an elevated temperature high enough to dissolve the polymer. Thereafter the mixture is cooled to effect crystallization of the polymer. A shaped article such as a film is formed during cooling. The film is subsequently stretched (either before or after solvent extraction) in at least one direction to produce the product. The product has a morphology characterized by a multiplicity of spaced, randomly dispersed non-uniform shaped, non-porous particles of the polymer. Adjacent particles throughout the material are separated from one another to provide micropores and which particles are connected to each other by a plurality of fibrils. No fluorocarbon polymers are included in the examples of crystallizable polymers. All the polymer/solvent systems disclosed are systems that exhibit solid (crystalline)/liquid phase separation behavior upon cooling from a one phase solution.
In applicants' prior application referred to above, a PFA or FEP melt blend film is extruded onto a quench medium device such as a quench roll. Both surfaces of this film are exposed to a short air gap distance before one side contacts the quench roll on which the entire film is subsequently quenched to effect phase separation. During the air gap residence period and part of the duration on the quench roll, it is believed that evaporation of the solvent from the surfaces of the film will occur before phase separation of the melt blend is completed. This is especially true of the film's surface that is not in direct contact with the quench roll surface during quenching because of its longer exposure period to ambient atmosphere at higher temperature. The evaporation is exacerbated by the very high temperature of the extrudate. The overall effect on the final resultant membrane is poor control of densification on the surfaces.
It would be desirable to provide a versatile process for producing microporous and open ultrafiltration membranes from PFA or FEP which permits control of the pore structure of one or both membrane surfaces and of the internal pore structure.